An electrocardiogram is a record of the electrical phenomena occurring in the heart to produce the coordinated contraction of the various chambers which assures an adequate circulation of blood through the body. The electrical fields thus generated follow the activity of the heart with time and are measured with electrodes at multiple sites on the body surface. The differences in electrical potential between strategically placed electrodes are recorded by electrocardiographs as voltage variations on the ordinate against time on the abscissa.
Although the electrical events in the heart can be displayed in many other ways, time-based electrocardiography is one of the most important diagnostic tools in clincal medicine. Normal and abnormal rhythms are identified and measured; the spread of electrical excitation in the heart muscle gives clues on a great number of impending, current or past pathological changes such as, e.g., localized ischemias, infarctions, necroses of tissue, etc. Recording the electrocardiogram with surface electrodes is virtually free of risk.
The interpretation of changes in the electrocardiogram waveform is the domain of expert cardiologists. Computer technology has been applied to electrocardiography with the objectives, (a), to facilitate and accelerate the evaluation of electrocardiograms in the hands of specialists as well as general practitioners, (b), to monitor the rhythm of the heart's excitation in high-risk patients automatically, (c), to transmit electrocardiograms rapidly and without distortion over the telephone to diagnostic centers and (d), to store the electrocardiograms in numerical form. All these objectives have been attained to a limited extent; rhythm abnormalities, in particular, are not detected with the help of machines that monitor the electrocardiogram continuously and can signal deviations as they occur.
The digitization of electrocardiograms for data extraction and processing is done, at the present time, by standard analog/digital conversion. This technique is nothing more than electronic curve tracing: positive and negative deviations from the isopotential line are sampled at a constant frequency and measured as discrete amplitude signals. Sampling and expressing the amplitude in discrete units carry the risk that the continuous signal waveform is not adequately represented. Any conversion error becomes visible when the digitized signal is again transformed, without further treatment, into its analog equivalent. The waveform reconstituted after A/D conversion follows the original record in steps, an averaging technique that frequently hides important information. The frequency response of non-dedicated transmission lines as well as available data storage capacities impose limitations on the sampling frequency and the number of bits representing the signal amplitude. For this reason, a number of compression methods have been devised which record only changes in the recurring signal beyond a set threshold value. It is known that these compressions can lead to serious errors.
The complex waveforms of the electrocardiogram have, to date, defied a mathematical characterization useful for the objectives mentioned above. Expressing the electrocardiogram as a Fourier series is possible but not practical for routine clinical use since a satisfactory representation in particular of the spikes in an electrocardiogram is achieved only by adding a great number of harmonics.